I bought two beautifully made Neo magnet bass drivers that came from a Mackie active speaker, so it is probably safe to assume they were made by RCF. The previous owner sold them as defective with scraping coils after an overload. However, when I received them, I carefully moved the cone up and down and could not detect any scrape. Actually, the wiggle rome for tilting the cone or moving the VC laterally was a lot bigger than in most new factory drivers I have com across.

The next test was to run 20 Hz. Both drivers exhibited a slightly metallic ticking sound but none of the scraping or metallic thud usually associated with a burned or deformed VC. It sounded a lot like tinsel slap or a poorly glued surround, but these phenomena usually start at a certain excursion whereas both my drivers exhibited it even with barely detectable drive level.

Thanks to the generously vented spider, I had a clear view of the VC. I was even able to pull all windings out of the gap thanks to a large maximum excursion. Their outside showed no signs of burned resin or other deformity. The coil was dull black with the exception of the upper winding (which happened to be on the inner layer), so I assumed it was blackened to improve heat dissipation.

Having checked all soft parts for loose glue joints, I finally removed a dustcap, but there were no loose particles or faulty joints underneath.

So pretty much the only explanation I have is that the resin that holds the turns in place on the former has come loose, allowing the individual windings to chatter against each other. I have seen older (15L200?) and fairly recent (L18S801) drivers where the turns have become unwound so that you can see a nest of wires through the sieve in the pole bore. Is it possible that my drivers are exhibiting a similar conditions except that the windings look like they are still securely in place?

If so, would a drop of fairly thin high temperature epoxy solve the situation? Should I let it cure cold or apply some DC current to speed curing?

Im no expert so dont shoot me down!
Even though the VC isnt showing signs of damage there could be an intermittent connection that will only show at full excertion... however, have you tried this 20 Hz test on a different amplifier ? it could be an amplifier clip or a poor audio source.

Furthermore, it could be the fact that the speaker is in open air (im assuming) try the speaker in an enclosure where the speaker is getting the correct amount of back pressure on the cone.

This can be caused by a tiny shard of ferrous metal in the gap, it moves around with the shifting magnetic field in the motor as it operates. Can sometimes be shifted by running the driver for a few hours at about 15Hz, otherwise it's cleaning the gap with sticky tape.

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A voice coil *may* be black (although I never saw one) but it will never be black for part of the winding and enamelled copper colour for another section of same winding.
In that case the copper coloured is the good one, and black is toast.
Meaning *the VC* is toast.
That metallic ticking sound is some loose part (caused by overheating or mechanical abuse), either a couple turns, VC partially separated from cone, you name it.
The speaker looks very good, but you'll have to recone it.
Hope you paid peanuts or nothing for it.
And no, dripping some adhesive down the coil will not repair it, sorry.

I have experienced this kind of problem (ferrous particle) before, and I could always feel the VC scraping, and the sound was also quite different. Also, what is the likelyhood of two such drivers feeling and sounding exactly alike?